CN variations in NGC 7006

¹ Max-Planck-Institut für Astronomie, Königsstuhl 17, 69117 Heidelberg, Germany
² UCO/Lick Observatory, University of California, CA 95064, USA

Corresponding author: D. Harbeck, harbeck@astro.wisc.edu

Received: 11 June 2003
Accepted: 29 July 2003

Abstract

Rotationally induced mixing with subsequent dredge-up of nucleosynthesized material is discussed as a second parameter of the horizontal branch morphology in globular clusters. CNO abundances have been proposed as tracers of the dredge up of processed material. NGC 7006is a prominent example of a second parameter GC: Its HB morphology is too red for its metallicity. We present spectroscopic measurements of CN molecular band strengths S(3839) and CH band CH(4300) strengths for 12 giants in NGC 7006to test rotationally-driven mixing as a second parameter in this cluster. Our observations reveal (i) a scatter in star-to-star CN absorption strengths with the same amplitude as seen in other GCs of the same metallicity, but different HB morphologies; (ii) a possible continuous distribution of CN absorption strength with a preference for CN-enriched stars, and (iii) a possible weak radial gradient in the number ratio of CN-strong and CN-weak stars. We argue against the hypothesis that CN-variations are directly correlated with the second parameter effect of the HB morphology. However, the small sample of stars measured in NGC 7006 prevents us from drawing firm conclusions. Finally, we identify one star of our sample as a foreground dwarf carbon star.